The dorsal cochlear nucleus (DCN) is one of the initial structures in the mammalian brainstem that processes sound, integrating multisensory and auditory nerve input. Multisensory modulation of auditory input is a basic feature of the DCN and may be a critical contributor to tinnitus, a common hearing impairment disorders characterized by the perception of sound without exterior auditory stimulus. Although multisensory modulation of auditory input in the DCN has been shown to be important for DCN function, the synaptic and cellular mechanisms underlying synaptic transmission from multisensory fibers to its primary targets within the DCN circuitry are not well known. Multisensory input to the DCN is relayed by mossy fibers (MF), whose primary synaptic targets are granule cells (GrCs). However, a large subset of GrCs receives input through an excitatory interneuron called unipolar brush cells (UBCs). UBCs have a single short dendrite that terminates in a brush-like structure and interdigitates with a single pre-synaptic terminal, formin an unusually large excitatory synaptic contact. A previous model suggests that prolonged entrapment of glutamate in the irregular synaptic cleft underlies the characteristic slow-decaying post-synaptic current (EPSC) of UBCs. The goal of this proposal is to investigate whether UBCs temporally amplify input from MF to GrCs for further integration with auditory input, and if so, what synaptic specializations promote such amplification. We propose that multivesicular release from MF terminals contributes to prolonged glutamate in the cleft and allows UBCs to provide temporal amplification of signals to GrCs. Aim 1 will determine the physiological postsynaptic response of GrCs to UBC mediated input These experiments will directly test for the first time the effect of UBCs input to GrCs using combined imaging and electrophysiological approaches in a GFP-tagged UBC mouse through 2- photon microscopy and patch-clamp recordings from connected UBC-GrC pairs. Aim 2 will determine the mode of exocytosis at the MF-UBC synapse. Preliminary data shows low variance in amplitude of evoked EPSCs in UBC upon MF stimulation with consistent synaptic depression, therefore suggesting a high probability of release the MF-UBC synapse. The proposed experiments will investigate the mode of release at these synapses with patch-clamp recordings and the analysis of the non-equilibrium inhibition of the post- synaptic receptors by low-affinity competitive antagonists coupled to multiple-probability fluctuation analysis. Through these methods the quantal size, the number of independent release sites and the probability of release at each site can be calculated. By investigating synaptic transmission through the main structure relaying multisensory input to the DCN, and the impact of UBCs to GrC activity, this proposal will provide the foundation for understanding how the DCN early processing of multisensory input affects integration with auditory input at the principal output cells. This will characterize the first cellular contributor pertaining to the modulation and triggers of somatic tinnitus at the DCN.